Rename 'free_submodules_sha1s()' to 'free_submodules_oids()' since the
function frees a 'struct string_list' which has a 'struct oid_array'
stored in the 'util' field.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>